Nanowire field effect transistors would allow 3-dimensional integration and thus a much higher device density than the current technologies. The key element of such a device is the semiconductor nanowire. So far, nanowires of a few semiconductor compounds produced by electroplating have been reported. However, nanowires of Si and Ge have only been produced by vacuum processes.
Moreover the previously reported nanowire structures made from electroplating are produced with a template, which defines the shape of the structures. As described in FIG. 1, the template (100) with nanopores (101) has a layer of conductive material (102) at one side, which carries the electroplating current and wire structures (103) are electroplated with the shape defined by the pores.
Germanium is a semiconducting material with higher mobility as compared to silicon. There have been some limited suggestions of electrodeposition of germanium onto metals. However, they have not been especially successful. These efforts may have not been thwarted due to the high reversible potential of Ge and the very low hydrogen overpotential on Ge surfaces. Therefore, all of the plating current can result from the proton reduction (side reaction) and no Ge plating can occur once the electrode surface is covered by Ge. So far, three approaches of Ge electroplating have been reported.
For instance, plating of germanium on metal substrates has been reported in alkaline aqueous solutions (see Fink et al., Journal of the Electrochemical Society, vol. 95, p. 80 (1948)) and in glycol solutions (see Szekely, Journal of the Electrochemical Society, vol. 98, p. 318 (1951) and U.S. Pat. No. 2,690,422 to Szekely). More recently, some studies have been reported directed nucleation studies in ionic liquid media. See Endres, Electrochemical and Solid State Letters, vol. 5, p. C38 (2002); Endres, Physical Chemistry and Chemical Physics, vol. 4, p. 1640 (2002) and Endres, Physical Chemistry and Chemical Physics, vol. 4, p. 1649 (2002).
In the aqueous solution approach, an extremely alkaline solution (pH>13) was suggested to minimize the proton concentration and therefore suppress the reduction of proton. This approach is not compatible with most microelectronics processes due to the extremely high pH of the electrolyte, which cause damages to most of the structures that are built from dielectrics such as silicon oxides.
In the non-aqueous solution approaches, where the glycol and ionic liquid solvents are used, the proton is either partially decreased or completely removed in the solution resulting in a much less side reaction. The techniques using ionic liquids suffer from the disadvantages of high viscosity and expenses.
Electroplating of germaniun on semiconductor substrates and especially on silicon has not been previously achieved, except as disclosed the co-pending US patent application entitled “Structures Containing Electrodeposited Germanium And Methods For Their Fabrication”, which is assigned to the same assignee and which discloses a method to produce epitaxial Ge thin film on semiconductor substrates by electroplating and solid state epitaxy. In said patent application, a method to electroplate free standing Ge nano-structure without being constrained in a template is disclosed.